This invention relates to pressure transducers and more particularly to methods for fabricating such transducers.
The prior art is replete with a number of transducer configurations which employ diaphragms and have located on the diaphragm piezoresistive elements. A plurality of such devices incorporate semiconductor diffusion techniques which operate to diffuse the transducers into the semiconductor substrate. Such devices are well known in the art. For example, reference is made to U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS issued on Apr. 4, 1972 to A. D. Kurtz et al. and assigned to the Assignee herein.
In any event, the prior art has been concerned with providing a pressure sensor where the strain gage is isolated from the diaphragm by means of a dielectric layer such as silicon dioxide. The techniques of the prior art disclosed the epitaxial growth of silicon on the silicon dioxide layer to provide a polycrystalline sensor or a polycrystalline substrate.
In U.S. Pat. No. 3,800,264 entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSINGS INCLUDING FABRICATION METHODS which issued on Mar. 26, 1974 to A. D. Kurtz et al and is assigned to the Assignee herein, there is shown a dielectrically isolated transducer. In this configuration, the polycrystalline layer serves as a support for the single crystal regions which are to be the strain sensors. The layer is provided by epitaxial growth techniques and incorporates therein a polycrystalline diaphragm structure. Based on this technique, other patents such as U.S. Pat. No. 3,858,150 entitled POLYCRYSTALLINE SILICON PRESSURE SENSOR issued on Dec. 31, 1974 to Gurtlar et al. and assigned to Motorola depicts a piezoresistive pressure sensor which is formed in a polycrystalline silicon layer. In this device, the polycrystalline silicon layer is deposited on an insulating etch stop layer of silicon nitride, which, in turn, is deposited over a substrate of monocrystalline silicon. The substrate is configured to support the silicon nitride layer by etching away the inner section.
U.S. Pat. No. 3,922,705 issued on Nov. 25, 1975 to Yerman and entitled A DIELECTRICALLY ISOLATED INTEGRAL SILICON DIAPHRAGM OR OTHER SEMICONDUCTOR PRODUCT shows a silicon diaphragm including a single crystal substrate bonded to a single crystal strain component with an intermediate insulating layer and a glass bonding layer. This patent requires the use of an intermediate glass layer for providing bonding and also employs epitaxial single crystal silicon regions. According to such techniques, U.S. Pat. No. 4,003,127 issued on Jan. 18, 1977 to Jaffe et al entitled POLYCRYSTALLINE SILICON PRESSURE TRANSDUCER depicts a transducer having a polycrystalline silicon diaphragm. The silicon diaphragm is vapor deposited on an etch resistant layer covering a surface of a wafer or base which is constructed from monocrystalline silicon.
In any event, it has long been an object of the transducer art to provide a single crystal silicon diaphragm having emplaced thereon single crystal silicon gages. Essentially, polycrystalline silicon is not as effective as single or monocrystalline silicon. The polycrystalline silicon does not possess the mechanical strength of the single crystalline silicon as well as the fact that the polycrystalline silicon tends to creep as it behaves in a more plastic way than single crystal silicon. Due to the nature of polycrystalline silicon, hysteresis occurs over temperature ranges at the crystal boundaries. Dimensional control of polycrystalline silicon is extremely difficult and normally in order to provide thin diaphragms, one requires thick wafers which are then lapped and polished to obtain proper control. On the other hand, single crystal silicon is inherently stronger than polycrystalline silicon and it does not exhibit any substantial creep or hysteresis problems. It is also preferable to work with certain types of etching on single crystal silicon in order to produce an optimum transducer. For example, in employing an anisotropic etch, one can achieve well defined sloping walls with single crystal silicon and hence, the mechanical geometry of the device is easily controlled over similar devices made and manufactured with polycrystalline silicon.
It is therefore an object of the present invention to provide a transducer structure which employs a single crystalline diaphragm and a single crystalline sensor separated from the diaphragm by an oxide layer. In this manner, one will provide both a single crystalline gage and a single crystalline diaphragm and hence, eliminate the disadvantages of polycrystalline structures. In order to fabricate such a device, one utilizes a process for implanting oxygen ions beneath the surface of a single crystal wafer and thereafter growing additional single crystal silicon on the surface of the ion implanted wafer. By employing this technique, one can provide a unique transducer configuration which has all the advantages of single crystal silicon.